1. Field of the Invention
The present invention is directed to an antenna structure for exciting a substantially homogeneous magnetic RF field and/or for receiving RF signals in a nuclear magnetic resonance tomography apparatus.
2. Description of the Prior Art
An antenna structure is disclosed in German OS No. 31 33 432, corresponding to U.S. Pat. No. 4,506,224, for use in a nuclear magnetic resonance tomography apparatus for exciting a substantially homogeneous magnetic RF field and/or for receiving corresponding RF signals. This known structure includes a hollow cylindrical conductor sheath which is transmissive for low frequencies for gradient magnetic fields, a plurality of conductor units forming at least one conductor pair and respectively consisting of one or more conductor elements combined into a group, the conductor groups extending in a longitudinal direction of the apparatus within the conductor sheath, reflectors at the end faces of the structure which reflect the waves of the RF field, and an external energy feed or reception means.
Imaging diagnostic methods have been developed in the field of medical technology wherein computed or measured integral resonance signals from nuclei of a selected chemical element of, for example, a human body or body organ, are analyzed. An image similar to an x-ray tomogram obtained by computer tomography can then be constructed from the three-dimensional spin density distribution and/or relaxation time distrubution thus acquired. Methods are generally known under the designations "nuclear magnetic resonance (NMR) tomography" or "zeugmatography." For nuclear magnetic resonance tomography, a strong magnetic field generated by a fundamental field magnet is required. This fundamental field must be highly uniform in the imaging, (examination) region. A body or organ to be examined is introduced into this region along an axis which usually coincides with the orientation axis of the fundamental magnetic field. The fundamental magnetic field is superimposed with stationary and/or pulse fields referred to as gradient fields. In order to excit individual atomic nuclei in the body or organ to execute a precessional motion, a special antenna installation is needed by means of which a RF magnetic alternating field (RF field) of adequate uniformatiy can be briefly excited. Under given conditions, this antenna installation can also be used for receiving the RF resonance signals thus elicited.
RF fields having high frequencies of 20 MHz and more can be excited or received with the RF antenna structure disclosed in the aforementioned U.S. Pat. No. 4,506,224. For this purpose, the antenna installation contains an outer hollow cylindrical element of non-magnetic material having good electrical conductivity. This antenna element is a conductor sheath surrounding a plurality of inner conductor elements forming at least one conductor pair. These inner conductor elements are disposed on an imaginary cylindrical surface around which the sheath is concentrically disposed at a predetermined distance. The .lambda./2 resonance condition for wave propagation having the aforementioned extremely high frequency is then set for the inner conductor elements (at least two in number), which are operated in the push-pull mode, and on the conductor sheath. Equiphase oscillating fields in the form of standing waves on the conductors are formed within the full examination volume of interest. To this end, the beginning and end of the two inner conductor elements are symmetrically electrically shorted by means such as, for example, predetermined capacitances which reflect the waves of the RF fields.
Because the outer antenna element forming a common conductor sheath around the inner conductor elements in the known structure is substantially transmissive for low-frequencies, the low-frequency gradient fields can propagate unimpeded in the examination volume.
In this known antenna structure, examination of the current distribution along the conductor elements at the desired .lambda./2 resonance shows a gradual decrease of the current toward the end faces of the conductors which is symmetrical relative to the center of the conductor. A current distribution curve can be seen which is roughly cosine-shaped. If one wishes to provide higher resonant frequencies or shorter wave lengths, using the same axial length of the antenna structure, a correspondingly smaller electrical shorting is required. This causes, however, an even more pronounced current drop along the conductor elements toward the end faces, such a pronounced current drop being undesired in view of maintaining a good field homogeneity.
A solution to this problem is suggested in German OS No. 33 47 597 wherein the non-homogenous current distribution can be at least partially compensated by bending the rod-shaped inner conductor elements to a barrel-shape, so that their distance from the outer hollow cylindrical conductor sheath is larger at the resonator ends than at the resonator center. The corresponding shaping of the inner conductor elements as well as their exact location within the outer conductor sheath, however, are relatively complex.